Question

How does Infrared Absorption Spectroscopy work$?$

Answer 1

Infrared absorption spectroscopy is a technique which is widely used in the field of chemistry to detect the presence of any functional group in a given molecule. It basically used the infrared radiations. Now you try to find out how radiations can be used to detect wavelength or frequency.

Complete step-by-step answer:
Infrared spectroscopy or vibrational spectroscopy is the measurement of the interaction of infrared radiation with matter by absorption, emission, or reflection. It is used to study and identify chemical substances or functional groups in solid, liquid, or gaseous forms. The method or technique of infrared spectroscopy is conducted with an instrument called an infrared spectrophotometer which produces an infrared spectrum.

Now, infrared absorption spectroscopy is a linear absorption technique in which intrinsic vibrational modes of the molecules are utilized to identify them. In fact, each chemical functional group is associated with its own set of vibrational modes with their corresponding frequencies of vibrations. In general, whole molecular frequency range can be classified into two regions: the region $600\,c{m^{ - 1}}\, - \,1600\,c{m^{ - 1}}$is called the fingerprint region and the region $1600\,c{m^{ - 1}} - 4000\,c{m^{ - 1}}$is called the functional group region.

The IR absorption spectra display absorption lines in these regions depicting vibrational transitions of molecules to register the presence of the molecules. This makes IR absorption an important phenomenon and IR absorption spectroscopy a valuable technique.

The vibrational transitions correspond to fundamental vibrations of chemical bonds which can be either bending or stretching $($symmetric or antisymmetric$)$. An IR spectrum can be visualized in a graph of infrared light absorbance on the vertical axis vs. wave number or frequency on the horizontal axis.

Note: Infrared Absorption Spectroscopy is not a very reliable method to identify a particular functional group. Hence various other spectroscopic methods were discovered for proper detection of a molecule, one of them being NMR spectroscopy which used nuclear magnetic resonance for detection of protons.